Taking wrinkling pictures

Wednesday July 11, 2007

We are all getting a little better at spin coating samples, but I for one lack consistency. It seems as though it would be a simple task, but indeed it is not. One of the most difficult aspects of this study is perfecting techniques.

This morning, we spin coated some glass slides with the 1.4% polystyrene (PS) solution (made on 5 July), and carried them over to Hasbrouck Hall, the Physics building at UMass. Here, Dr. Menon has given Jiangshui some space for his Olympus stereo microscope, camera, and computer for recording wrinkling patterns on the PS films. In the Hasbrouck lab, we then used a template and a small cutting device (resembling a diamond tipped pen for etching) to score the film. To remove the scored circle from the glass slide, a water bath is prepared in a small Pyrex dish (maybe 250 ml of water), and the glass slide is slowly lowered into the bath at a 45 degree angle. The water comes between the glass slide and the scored PS circle; the PS is more attracted to the water than the slide, so the circle of film lifts off of the slide and floats on the surface of the water. You can't really see the circle of PS film (it is only 100 nm thick after all), but you can see the difference in the reflection of the light off the surface of the film and the surface of the water. You really have to move your head around and catch the angle just right to find the floating film.


A microsyringe is filled with water, and one 0.2 ul drop of water is placed in the center of the circular floating film. A digital camera is mounted on the microscope, and a small metal probe is used to gently center the film and droplet into the view field of the camera. Photographs are taken under low and high magnification; the film's edge is seen, along with the central drop, under low magnification. The magnification is then increased so that the edge of wrinkling is evident, and a second picture is taken. A second 0.2 ul drop is placed precisely on top of the first drop, increasing its volume and therefore the effect of the drop on the film. The second (and all subsequent) drops must be delivered exactly perpendicular to the previous drop. The additional drops are almost pulled off of the tip of the microsyringe onto the existing drop by the cohesive properties of the water molecules.

Pictures are taken at two magnifications, and drops are added in 0.2 ul increments until a total volume of 1.4 ul has been reached. The experiment is repeated with other prepared films. We have done this many times to get consistent data.



After transferring the pictures from the digital camera to the computer, we upload them on a flash drive and return to the Conte Center computer room to analyze the data using Image J software, which allows us to draw a circle around the film in the picture taken under low magnification which allows the software to calculate the pixel area which will be compared to the pixel area of the water drop. This is used to establish ratios. A second circle is drawn around the circumference of the drop, and its pixel area is determined. Under high magnification, a circle is drawn around the boundary of the wrinkling, and a circle is drawn around the drop and the pixel area is calculated for each. The data obtained from Image J is then entered into a program called Origin, which is similar to an Excel spread sheet and uses the data from Image J to establish ratios among the measurements and calculate the length of the wrinkles.

Wrinkle length and wrinkle number are a function of the thickness of the film of the substance under study, in our case polystyrene.